The present invention relates to an apparatus for introducing oxygen gas into a vacuum chamber, and more particularly to an apparatus for introducing oxygen gas into a vacuum electron optical column of an electron beam instrument such as an electron microscope, an electron beam lithography system, or the like in which an electron gun having a thermal field emission cathode that needs a minute partial pressure of oxygen gas is used as an electron source.
A method for introducing oxygen gas into a vacuum chamber without an oxygen gas cylinder is known in which the oxygen gas may be introduced into the vacuum chamber by heating a thin silver plate or cylinder located for partitioning the atmosphere or oxygen atmosphere and the vacuum chamber from each other.
On the other hand, in electron beam applied systems such as a scanning type electron microscope and an electron beam lithography system, a thermal field emission cathode must be used in order to cause an electron beam source to have a higher brightness to make a converged beam fine and to increase the beam current. In particular, a minute oxygen gas partial pressure is often needed when an electron gun is used. That electron gun is constructed so that at least one of metals of titanium (Ti), zirconium (Zr) and hafnium (Hf) that are capable of inducing a large current and are superior in current stability is absorbed through the oxygen and adhered to a monocrystalline tip surface of tungsten or molybdenum having a directional property of [100] to form a substantially mono-atom layer (see Japanese patent unexamined publication No. 61,733/81). In this case, if the oxygen gas introducing system using the above-described thin silver plate or cylinder is used, it is unnecessary to use an oxygen gas cylinder that is large in physical dimension and that needs an adjusting valve for introducing a minute amount of the oxygen gas, and hence, it is easy to introduce the oxygen gas into a desired section.
The oxygen gas introducing apparatus using the silver cylinder has heretofore been applied as disclosed in Japanese patent unexamined publication No. 103,968/75. Namely, as shown in cross-section in FIG. 1 a current is supplied to a heating wire 2 arranged around an outer periphery of a silver cylinder 1 which extends from a vacuum mirror wall 5 to the atmosphere, so that the wire 2 is heated. As a result, the atmosphere 3 existing between the silver cylinder 1 and a cover 4 or the oxygen gas of the oxygen gas atmosphere having a pressure that is substantially equal to the atmospheric pressure is introduced into the vacuum chamber.
However, such a structure suffers from problems such that (1) a heating electric power required for heating the wire is large and (2) an oxygen introduction efficiency is decreased. If the amount of the introduced oxygen is increased in order to solve the latter problem (2), the heating power for heating the wire will be further increased. This causes another problem that (3) the silver cylinder 1 would be locally molten to induce a serious accident.
Thus, the conventional oxygen gas introducing apparatus disposed in the vicinity of the thermal field emission type electron gun chamber involves various practical problems; that is, the problem (1) causes a obstacle of electromagnetic induction in the electron gun, the problem (2) makes it difficult to control the necessary oxygen gas partial pressure, and the problem (3) leads to the break-down of the cathode with the electric discharge. Effforts made to cope with these problems in the conventional apparatus are insufficient.